Several stores around me have marked down Mobil 1 5W-40 for Diesel trucks to $5.
Example zip code with it in stock is 75071.
There is also a Mobil oil rebate discussed in this thread
The actual rebate form is here [mobil.com]
and this diesel oil qualifies for a $12 rebate, making this potentially a $7 money maker (assuming you only buy one)
Brickseek link [brickseek.com]
Walmart link [walmart.com]
(for reference only)